UNIVERSITY    OF    CALIFORNIA     AGRICULTURAL    EXPERIMENT  STATION 
COLLEGE   OF  AGRICULTURE  "NJ"  '°E  WHEELER'  '«■■«■« 

THOMAS    FORSYTH    HUNT,    Dean  AND  DIRECTOR 

BERKELEY  h.  e.  van  norman,  vice-director  and  dean 

University  Farm  School 


CIRCULAR  No.  213 
May,  1919 

EVAPORATORS  FOR  PRUNE  DRYING 

By  W.  V.  CRUESS 


CONTENTS 

PAGE 

A.  Theory  of  Evaporation  ... . 2 

B.  General  Principles  of  Various  Types  of  Evaporators  ... 3 

1.  Defects  of  Some  Present  Evaporators 3 

2.  Types  of  Evaporators 4 

(a)  Air  Blast  Evaporator , ,..  4 

( b )  Evaporators  Depending  upon  Ventilators  for  Air  Circulation  4 

(c)  Vacuum  Evaporators 5 

C.  Construction  and  Use  of  Evaporators 5 

1.  The  Oregon  Tunnel  Evaporator 5 

(a)   General  Specifications  of  a  Tunnel  Evaporator 7 

(fe)   Cost  of  Tunnel  Evaporator 9 

(c)  Discussion  of  Oregon  Tunnel  Evaporator 10 

(d)  Operation   of   the   Evaporator 14 

2.  Air-Blast  Evaporator  14 

(a)  General  Specifications  of  Air-Blast  Evaporator 14 

(fe)   Cost  of  Air-Blast  Evaporator 17 

(c)  Discussion  of  Air-Blast  Evaporator 18 

(d)  Operation  of  Air-Blast  Evaporator 21 

3.  The  Young  Evaporator 22 

(«)   General  Specifications  of  Young  Evaporator 22 

(b)  Cost  of  Young  Evaporator 24 

(c)  Discussion  of  Young  Evaporator 24 

(d)  Operation  of  Young  Evaporator 25 

4.  Other  Evaporators  25 

(a)   Kiln  Evaporator  25 

(ft)   Watsonville  Stack  Evaporator 27 

(c)  The  Anderson  Evaporator 27 

(d)  Commercially  Built  Evaporators 28 

Summary  and  Conclusions 28 

Eecommendation 30 


In  years  when  early  rains  occur  during  the  prune-drying  season, 
loss  of  fruit  sometimes  occurs  because  of  the  failure  of  the  usual 
sun-drying  methods.  Various  methods  of  supplementing  sun-drying 
during  such  emergencies  are  in  use,  one  of  the  best  being  that  of 
sulfuring  described  in  Circular  211. 

Better  results  are  obtained,  however,  under  such  circumstances 
by  the  use  of  dryers  or  evaporators.  It  is  also  often  desirable  to 
supplement  sulfuring  with  these  evaporators.  The  initial  cost  of  an 
evaporator  may  appear  large  but  the  loss  during  one  season,  such  as 
that  of  1918,  would  be  several  times  greater  than  this  cost.  The 
number  of  evaporators  in  the  prune-growing  districts  of  the  state  is 
too  small  and  many  of  those  in  use  are  inefficient. 

This  circular  gives  the  results  of  a  study  of  the  principal  evapora- 
tors in  California  and  of  the  literature  on  the  subject.  It  was  found 
that  fundamental  principles  of  artificial  evaporation  have  been 
neglected  in  most  of  the  evaporators  now  used  in  our  prune-drying 
yards.  As  a  result  of  our  investigations,  we  are  enabled  to  recommend 
remedies  for  some  of  these  defects  and  to  suggest  a  better  design  for 
new  evaporators. 

A  short  discussion  of  the  theory  of  evaporation  will  give  a  clearer 
understanding  of  the  general  principles. 

A.    THEORY   OF   EVAPORATION 

Evaporation  is  the  change  of  water  from  the  liquid  to  the  gaseous  or  vapor 
state.  It  is  influenced  by  heat  and  by  dryness  of  the  air.  The  heat  used  may 
be  that  occurring  naturally,  as  a  result  of  radiation  from  the  sun,  or  that  obtained 
by  the  burning  of  fuel.  The  moisture  evaporated  by  heat  is  absorbed  and  carried 
away  by  the  atmosphere.  The  higher  the  temperature  the  more  moisture  the 
atmosphere  can  hold.  At  a  temperature  of  101°  F.  it  will  require  349  cubic  feet 
of  the  atmosphere  to  absorb  one  pound  of  water  vapor;  at  128°  F.  the  same 
volume  of  air  would  hold,  at  saturation,  two  pounds  of  water  vapor ;  the  same 
volume  of  air  at  156°  F.  would  absorb  four  pounds  of  water  vapor.  Every 
27°  F.  rise  in  temperature  doubles  the  moisture-absorbing  power  of  the  atmosphere. 

The  atmosphere  is  always  more  or  less  humid;  that  is,  contains  some  water 
vapor.  The  less  moisture  it  contains,  the  more  it  can  absorb  at  a  given  tempera- 
ture. A  volume  of  349  cubic  feet  of  air  at  101°  F.  and  10  per  cent  humidity 
can  still  absorb  .9  pound  of  water  vapor,  while  the  same  volume  of  air  at  101°  F. 
and  50  per  cent  humidity  can  only  absorb  .5  pound  of  water  vapor.  In  order 
that  evaporation  may  be  rapid  and  continuous,  the  saturated  air  must  be  removed; 
therefore  movement  of  drier  air  through  the  evaporator  is  essential. 

As  the  evaporation  of  moisture  requires  heat,  the  heat  absorbed  from  the  air 
during  evaporation  causes  a  lowering  of  the  temperature  of  the  air.  This  drop 
in  temperature  lowers  the  moisture-absorbing  power  of  the  air  so  that  the 
amount  of  moisture  that  the  air  will  absorb  is  less  than  that  which  it  would  absorb 
if  the  temperature  could  be  held  constant  at  the  higher  level.     Moreover,  as  the 


atmosphere  approaches  the  point  of  saturation,  evaporation  becomes  slow,  so 
that  it  is  not  economical  to  try  to  utilize  all  of  its  drying  power.  This  results 
in  a  loss  of  a  large  amount  of  heat.  Some  heat  is  taken  up  by  the  article  dried, 
the  walls  of  the  evaporator,  and  by  other  articles  that  come  in  contact  with  the  air. 

If  air  is  taken  in  at  90°  F.,  heated  to  150°  F.,  and  passes  out  of  the  evaporator 
at  120°  F.,  the  air  is  heated  60°  F.  and  drops  only  30°  F.  In  this  case  50  per  cent 
of  the  heat  is  unused. 

Tests  by  heating  and  ventilating  engineers  indicate  that  on  the  average  only 
about  40  per  cent  of  the  heat  furnished  to  an  evaporator  is  utilized.  Theoretically, 
the  heat  generated  by  one  pound  of  oil  will  evaporate  about  18  pounds  of  water, 
but  in  practice  it  evaporates  only  about  7Y±  pounds. 

Other  things  being  equal,  the  rate  of  evaporation  is  more  or  less  proportional 
to  the  temperature  and  the  volume  of  air  passing  through  the  evaporator.  How- 
ever, it  is  possible  by  raising  the  temperature  to  reach  a  point  where  the  water 
from  the  interior  of  the  fruit  can  not  diffuse  to  the  surface  as  rapidly  as  it  is 
absorbed.  This  results  in  "case  hardening"  or  drying  out  and  partial  sealing  of 
the  surface  of  the  fruits,  with  a  decrease  in  the  rate  of  drying. 

There  is  a  temperature  above  which  all  fruits,  when  dry,  deteriorate  rapidly. 
For  prunes  this  temperature  is  believed  to  be  about  150°  F.  It  is  spoken  of  as 
the  ' '  critical  temperature. ' ' 

B.     GENERAL    PRINCIPLES    OF   VARIOUS   TYPES    OF    EVAPORATORS 

1.  Defects  of  Some  Present  Evaporators. — A  large  volume  of  air 
of  low  humidity  is  necessary  for  rapid  evaporation.  Many  evaporators 
do  not  provide  this  condition.  In  some  evaporators,  the  air  is  made 
to  circulate  over  the  fruit  again  and  again  with  the  result  that  it 
becomes  saturated  with  moisture  and  merely  cooks  the  fruit  without 
drying  it.  Such  evaporators  provide  no  inlet  for  outside  air  and  no 
outlet  for  the  saturated  air.  In  other  evaporators  the  apertures  for 
entrance  of  fresh  air  and  escape  of  spent  air  are  altogether  too  small 
in  proportion  to  the  size  of  the  evaporator. 

Often  the  air  circulation  in  the  evaporator  is  not  uniform;  trays 
of  fruit  near  the  floor  drjdng  more  or  less  rapidly  than  those  at  other 
levels.  This  makes  shifting  of  the  trays  necessary  and  increases  the 
labor  cost.  In  other  driers,  large  channels  are  left  beside  the  stacks 
of  trays.  The  air  tends  to  flow  through  these  spaces  rather  than  over 
the  fruit. 

In  some  evaporators,  the  heat  supply  is  very  inadequate.  If  the 
air  is  passed  rapidly  its  temperature  will  be  low;  if  passed  slowly  its 
temperature  may  be  high  enough  but  its  volume  will  be  small.  In 
either  case  the  evaporation  will  be  small.  The  furnace  or  other  heat- 
ing device  should  produce  enough  heat  and  the  radiating  surface 
should  be  large  enough  to  deliver  this  heat  to  the  air  without  waste. 
The  heat  generated  in  the  furnace  should  not  be  allowed  to  escape 
through  the  smokestack.     Where  flues  are  used  to  heat  the  air,  they 


have  in  most  cases  altogether  too  little  radiating  surface.  Fires  have 
been  caused  by  overheating  such  flues  in  an  attempt  to  force  them 
beyond  their  capacity. 

Most  California  evaporators  are  equipped  with  poor  thermometers 
or  none  at  all.  Accurate  thermometers,  if  possible,  recording  ther- 
mometers, should  be  installed  in  order  that  the  operator  may  know 
the  correct  or  exact  temperature  inside  the  evaporator. 

2.  Types  of  Evaporators. — There  are,  in  general,  three  types  of 
evaporators  for  fruits,  viz.,  {a)  forced  draft  or  air  blast  evaporators, 
(&)  evaporators  depending  on  ventilators  for  air  circulation,  and  (c) 
vacuum  evaporators.  The  last  named  type  is  not  in  common  use  for 
fruits  and  is  too  expensive  for  general  fruit  evaporation. 

(a)  Air  Blast  Evaporator:  Usually  this  evaporator  consists  of  a 
long  horizontal  or  nearly  horizontal  chamber  or  tunnel  in  which  the 
fruit  is  placed  for  drying.  A  fan  blows  or  draws  air  through  a  heat- 
ing system  located  at  one  end  of  the  tunnel  and  forces  the  heated  air 
through  the  drying  chamber. 

The  fan  may  be  of  the  positive  blower  type,  in  which  case  the  air 
heater  and  fan  are  located  at  the  same  end  of  the  evaporator ;  or  it 
may  be  of  the  exhaust  or  suction  type,  in  which  case  the  air  is  drawn 
through  the  evaporator  and  the  air  heating  system  is  located  at  the 
end  opposite  the  fan.  The  positive  blower  type  is  most  common,  but 
the  exhaust  fan  is  believed  to  produce  a  more  uniform  current  of  air 
throughout  the  length  of  the  evaporator. 

The  drying  chamber  may  connect  with  a  ventilator  which  tends  to 
increase  the  flow  of  air.  The  ventilator  in  such  installations  is  of 
secondary  importance,  provided  it  is  not  so  narrow  that  it  retards 
rather  than  aids  air  flow. 

The  air  heating  system  is  most  often  composed  of  steam  coils  over 
which  the  air  is  drawn  or  blown  by  a  fan.  The  temperature  of  the 
air  can  be  closely  regulated  by  controling  the  pressure  or  amount  of 
steam  used.  Another  system  in  common  use  consists  of  a  furnace  and 
large  flues  enclosed  in  a  fire-proof  room.  Air  is  drawn  over  the  flues 
by  a  fan.  This  type  of  construction  is  cheaper  than  that  employing 
steam  coils  and  can  be  made  to  operate  as  efficiently. 

(&)  Evaporators  Depending  upon  Ventilators  for  Air  Circulation: 
The  design  of  this  type  varies  greatly,  but  air  circulation  is  accom- 
plished in  all  by  means  of  ventilators  and  natural  draft.  The  venti- 
lator consists  of  a  tall  stack  connected  to  the  drying  compartment.  It 
produces  a  draft  through  the  evaporator,  in  the  same  way  that  an 
ordinary  furnace  stack  or  stovepipe  creates  a  draft  through  a  furnace 
or  stove.    The  taller  the  ventilator  the  stronger  the  draft. 


The  heating  system  is  located  below  the  material  to  be  dried.  Thus, 
the  heated  air,  being*  lighter  than  the  surrounding  outside  air,  rises 
through  the  drying  chamber  and  after  taking  up  moisture  escapes 
through  the  ventilator.  The  heat  may  be  supplied  by  a  furnace  and 
flues  or  by  a  steam  plant  and  steam  pipes.  The  former  system  is  the 
one  most  commonly  employed. 

Air  is  admitted  at  the  bottom  of  the  air  heating  chamber  and  the 
amount  admitted  is  so  regulated  that  the  desired  temperature  is  main- 
tained in  the  evaporator.  For  a  given  temperature,  the  rate  of 
evaporation  will  be  proportional  to  the  air  flow  within  certain  limits. 

The  ordinary  hop  kiln,  the  Oregon  prune  tunnel,  the  steam  coil 
cabinet  evaporator,  and  the  stack  evaporator  are  all  of  this  type. 
Examples  of  these  are  described  later. 

(c)  Vacuum  Evaporators:  These  are  used  in  drying  certain  chem- 
icals and  substances  easily  injured  by  heat  or  air.  Under  a  vacuum, 
evaporation  proceeds  at  a  much  lower  temperature  than  under 
atmospheric  pressure.  Fruit  can  be  evaporated  very  rapidly  at 
100°  F.  in  a  vacuum  evaporator.  Oxidation  by  air  which  takes  place 
in  ordinary  evaporators  is  very  greatly  reduced.  Consequently,  a 
product  of  fresh  flavor  and  appearance  can  be  produced  by  this 
method.  The  construction  of  vacuum  evaporators  is,  however,  very 
expensive  and  a  great  deal  of  skill  and  experience  is  necessary  for 
their  successful  operation.  They  are  not  recommended  for  prune 
drying. 

C.     CONSTRUCTION    AND    USE    OF    EVAPORATORS 

The  style  and  size  of  evaporator  to  be  constructed  will  vary  with 
the  size  of  the  dry-yard  and  with  the  personal  preference  of  the 
builder.  The  Oregon  tunnel  evaporator  has  been  found  very  satis- 
factory in  Oregon  and  will  probably  answer  the  needs  of  the  dry- 
yard  of  average  size  as  well  as  any  other  form  that  could  be  built. 
For  the  large  dry-yard,  an  air-blast  type  of  dryer  will  probably  be 
most  useful.  For  the  small  yard  an  inexpensive  evaporator,  such  as 
the  Young  evaporator  or  the  New  Way  evaporator,  is  desirable. 

Descriptions  and  specifications  for  (1)  an  Oregon  prune  tunnel 
evaporator,  (2)  an  air  blast  evaporator,  and  (3)  a  small  stack 
evaporator  are  given  below. 

1.  The  Oregon  Tunnel  Evaporator. — This  evaporator  is  used  very 
extensively  in  Oregon  and  Washington  for  drying  the  fresh  fruit  as  it 
comes  from  the  orchard. 

In  a  general  waj^,  the  tunnel  evaporator  may  be  described  as  a 
series  of  parallel,  nearly  horizontal  narrow  chambers,  above  a  firepit. 


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The  trays  of  fruit  enter  the  upper  and  cooler  end  of  each  tunnel  on 
slides  or  runways  and  are  taken  from  the  lower  and  hotter  end  of 
the  tunnel  when  dry ;  the  tray  entering  the  upper  end  displaces  a  tray 
of  dry  fruit  at  the  lower  end. 

This  evaporator  is  of  moderate  cost,  is  not  difficult  to  operate,  is 
efficient  in  its  use  of  heat  and  gives  a  good  dried  product. 
(a)   General  specifications  of  a  tunnel  evaporator: 

(1)   Six  tunnels,  20'  X  36%"  X  6'. 
(2) Slope  of  tunnels,  2"  per  V. 

(3)  Walls  of  tunnels  of  2"  X  4"  studs,  covered  with.  T.  and  G. 

(4)  Floors  of   tunnels  of   steet  iron;    ordinary  heavy   gauge  galvanized  iron 
will  answer. 


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Fig.  2. — Plan  of  second  floor,  showing  location  of  tunnels. 


(5)  Tunnels  to  be  supported  on  6"  I  beams  which  rest  on  brick  or  cement 
walls  of  the  furnace  room. 

(6)  Tray  runways,  1%"  X  1%".  Six  inches  from  center  to  center  between 
first  pair  of  runways  at  top  of  tunnel;  *4"  decrease  in  this  distance  between  each 
two  runways  until  3%"  is  reached  for  runways  near  bottom  of  tunnel.  Telephone 
insulators  may  be  added  to  runways  to  facilitate  sliding  of  trays.     (See  fig.  3.) 

(7)  Ventilator,  30"  X  36",  for  each  tunnel,  and  16'  in  height  from  top  of 
tunnel,  and  equipped  with  damper,  as  shown  in  figure  5. 

(8)  Doors  at  each  end  of  tunnel  of  beaver  board  or  transite  board  on  light 
frame.  Doors  to  slide  upward  and  to  be  balanced  by  window  weights  on  pulleys 
above  tunnels.     (See  fig.  3.) 

(9)  Opening  3'  wide  at  lower  end  of  each  tunnel  floor  and  fitted  with  sliding 
door,  as  shown  in  figure  5. 

(10)  Finishing  chamber  equal  to  length  of  one  tray  and  height  of  three  tray 
runways  at  top  of  tunnel  at  lower  end.     Chamber  has  no  floor.     (See  fig.  4.) 

(11)  Heating  chamber  22'  long,  10'  wide,  12'  high  at  one  end  and  9'  at  the 
other.  Walls  of  8"  concrete  or  of  brick  or  tile.  Eoof  of  heating  chamber  is 
formed  by  floor  of  tunnels.  Floor  of  earth  or  cement  as  desired.  (See  figs.  1 
and  5.) 


(12)  Four  openings,  3'  X  1%'  in  walls  of  heating  chamber  6"  from  ground. 
Fitted  with  adjustable  sliding  doors  to  regulate  air  intake.     (See  figs.  1  and  5.) 

(13)  Furnace  8'  X  2y2'  X  2'.  Walls  and  floor  of  fire  brick.  Top  of  W 
sheet  iron  arched.  Furnace  door  to  be  flush  with  outer  wall  of  furnace  room. 
Other  end  of  furnace  to  connect  to  16"  pipe.  Furnace  to  be  equipped  with  large 
rotary  crude  oil  or  stove  distilate  burner. 

(14)  16"  pipe  reducing  to  13"  pipe  to  be  arranged  from  furnace  back  and 
forth  across  heating  chamber.  Pipe  to  slope  gently  upward  throughout  to  stack 
connection,  but  not  to  approach  nearer  than  30"  to  floor  of  tunnels.  About  125 
feet  of  pipe  should  be  used.  (See  fig.  5  for  general  plan  of  piping.)  Best  to 
have  pipe  at  least  48"  below  floor. 


Fig.  3. — Lower  end  of  tunnel,  showing  construction  of  sliding  doors  of  beaver 
board  and  of  tray  runways.  Note  that  trays  are  offset  and  that  the  upper  trays 
extend  farther  than  the  lower.  (After  Bulletin  145,  Oregon  Agricultural  Experi- 
ment Station,  by  Lewis,  Brown  and  Barss.) 


(15)  Thermometers  must  be  placed  in  tunnels  at  lower  and  upper  ends;  a 
recording  thermometer  at  lower  end  of  one  tunnel  is  very  useful. 

(16)  Sorting  table,  15'  X  3',  as  shown  in  figure  2. 

(17)  Trays  of  1/4"  or  %"  mesh  hardware  cloth  between  two  frames  of  %" 
by  iy2"  spruce  slats.  Frames  for  36%"  tunnel  to  be  36"  X  48"  or  36"  X  36"  and 
braced  by  two  %"  X  %"  crosspieces.  There  should  be  about  100  trays  for  each 
tunnel  to  permit  continuous  operation  of  the  evaporator. 

(18)  Building  to  be  of  barn  construction  and  of  general  dimensions  shown 
in  figures  9  and  10.  To  consist  of  two  furnace  rooms  on  ground  floor ;  a  receiving 
room  or  receiving  porch  about  32'  X  12';  a  sloping  space  31'  6"  wide  X  21'  long 
for  tunnels,  permitting  an  alleyway  5'  wide  at  each  side  of  tunnels;  a  room 
10'  X  31%'  at  lower  end  of  tunnels  for  receiving  trays  from  evaporator;  roof 
of  rubberoid  roofing,  galvanized  iron,  or  shingles. 


(19)  Alternative  construction  for  building:  Some  evaporators  are  housed  in 
galvanized  corrugated  sheet-iron  buildings.  This  type  of  construction  reduces 
the  fire  risk. 

(&)  Cost  of  Tunnel  Evaporator:  The  cost  will  depend  very  largely 
upon  the  quality  of  materials  used  and  upon  how  much  of  the  con- 
struction is  done  by  the  owner.  A  seven-tunnel  evaporator  was  built 
by  L.  J.  Ecldens  of  Dundee,  Oregon,  for  $1300  cash  in  1916.  It  has 
a  capacity  of  nine  tons  at  each  loading.  This  cost  is  exceptionally  low 
because  the  owner  did  not  add  the  value  of  his  own  labor  to  the  cost 
of  construction.  This  evaporator  is  described  in  Bulletin  145  of  the 
Oregon  Agricultural  Experiment  Station  at  Corvallis. 


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Fig.  4. — A,  cross-section  of  tunnels;  B,  cross-section  of  tunnels,  showing  finishing 

chamber. 


J.  S.  Caldwell  estimates  the  cost  of  a  nine-tunnel  evaporator  of 
first  class  construction  at  $3828.  This  evaporator  is  described  in 
Bulletin  148  of  the  Washington  Agricultural  Experiment  Station,  at 
Pullman. 


Caldwell 's  estimates  of  costs  are  as  follows : 

1.  Concrete  work,  105  cu.  yds.,  at  $8 $840.00 

2.  Iron  rods  for  reinforcements,  2000  lbs.,  at  6c 120.00 

3.  Roofing,  3-ply  asphalt,  3000  sq.  ft.,  at  $2.50  per  square 75.00 

4.  Eafters,  1500  ft.,  2"  X  6",  at  $20  per  1000 30.00 

5.  Sills  and  joists,  1600  ft.,  6"  X  6",  at  $20 32.00 

6.  Framing  for  ventilators,  bins,  and  tunnels,  4800  ft.  of  2"  X  4",  at 

$20  per  1000 96.00 

7.  Flooring,  5500  ft.,  at  $28  per  1000 154.00 

8.  Ship  lap  for  sheathing  roof,  making  bins  and  ventilator,  8000  ft. 

at  $16  per  1000 128.00 


10 

9.  Chimneys,  concrete  base  and  brick  flues 60.00 

10.  Furnaces,  three  with  fire  brick  lining,  complete 60.00 

11.  Piping,   720   ft.,  9-inch,  at  10c 72.00 

12.  Kunways  for  trays,  800  ft.,  1"  X  %",  at  $20 16.00 

13.  Metal  sheets  for  floor  of  tunnels,  475  sq.  ft.  at  $4  per  square 20.00 

14.  Labor    of    constructing   tunnels 150.00 

15.  Twenty  doors  for  tunnels,  at  $4,  ten  outer  doors  at  $4 120.00 

16.  Twenty  windows  at  $1.60 35.20 

17.  Lumber  for  paring  tables,  spreading  tables,  chutes,  conveyors,  and 

bleacher  for  apples,  1200  ft.,  at  $28  per  1000 33.60 

18.  Labor  in  building  chutes,  etc 135.00 

19.  Master  carpenter,  at  $5.50;   ordinary  carpenter,  at  $4.50  per  day. 

Carpenters'  labor  on  floors,  bins,  ventilators,  windows,  etc 320.00 

20.  Metal  parts  for  conveyor,  bleacher,  and  shafting  and  belting 200.00 

21.  Minor  hardware,  nails,  hinges,   etc 80.00 

22.  Trays,  1400  at  75c 1050.00 


Total $3828.80 

For  a  prune  evaporator  for  California,  items  17,  18,  and  20  would 
be  omitted.  This  would  reduce  the  total  to  $3460.20.  This  evaporator 
would  hold  at  one  charge  about  10%  tons  of  fruit. 

A  six-tunnel  evaporator  would,  based  on  two-thirds  of  the  above 
estimate  for  a  nine-tunnel  evaporator,  cost  about  $2250,  although  a 
six-tunnel  evaporator  would  cost  more  than  two-thirds  as  much  as  a 
nine-tunnel  evaporator,  because  the  latter  does  not  require  a  building 
one-half  larger  than  the  six-tunnel  type. 

Probable  $2500  is  a  safe  estimate  of  the  cost  of  a  six-tunnel  prune 
evaporator.  The  evaporator  building  is  very  useful  for  storage  of 
trays,  lug  boxes,  and  machinery  during  the  slack  season. 

(c)  Discussion  of  Oregon  Tunnel  Evaporator:  Being  built  upon 
the  unit  system,  a  single-unit  small  evaporator  of  one  to  three  tunnels 
may  be  built,  and  a  large  evaporator  may  be  readily  made  by  increas- 
ing the  number  of  units.  Where  a  small  evaporator  of  three  tunnels 
or  less  is  built,  it  will  often  be  possible  to  utilize  an  existing  building 
to  house  the  evaporator.  A  small  evaporator  of  this  type  is  fully 
described  in  Farmers'  Bullein  984,  a  copy  of  which  may  be  obtained 
free  of  charge  from  the  United  States  Department  of  Agriculture, 
Washington,  D.  C. 

The  individual  tunnels  should  be  separated  from  each  other  to 
obtain  the  most  satisfactory  and  uniform  air  circulation.  The  floor 
of  the  tunnel,  except  for  the  three-foot  opening  at  the  lower  end, 
should  be  covered  with  sheet  metal.  The  floor  is  omitted  in  some 
tunnels,  but  this  construction  results  in  improper  circulation  and  in 
overheating  of  the  lower  trays. 


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The  tray  runways  may  be  seen  in  figure  3.  If  these  runways  are 
not  smooth  and  perfectly  straight  it  requires  undue  force  to  move  the 
trays.  Telephone  insulators  or  small  casters  nailed  to  the  runways 
facilitate  sliding  of  the  trays.  A  small  cleat  at  the  lower  end  of  each 
runway  will  hold  the  last  tray  in  place. 

If  the  upper  runways  are  farther  apart  than  the  lower,  air  cir- 
culation is  made  more  uniform.  For  the  same  reason  the  upper  trays 
should  extend  farther  over  the  tunnel  throat  than  the  lower  trays. 
(See  fig.  3.) 

The  ventilator  is  a  very  important  feature  of  the  evaporator 
because  upon  its  operation  depends  the  air  flow  through  the  tunnel. 
It  should  be  tall  enough  to  give  a  good  draft;  a  velocity  of  air  flow 
of  about  600  feet  or  more  per  minute  is  desirable.  The  ventilator 
extends  across  the  upper  end  of  all  of  the  tunnels  but  should  have 
partitions  separating  it  into  a  flue  for  each  tunnel.  The  ventilator 
should  be  at  least  30  inches  wide  and  15  to  20  feet  tall  to  give  satis- 
factory results.  Each  flue  of  the  ventilator  should  be  fitted  with  a 
clamper  that  can  be  easily  regulated,  as  shown  in  figure  5. 

The  air  heating  system  must  be  large  enough  to  heat  a  large  volume 
of  air  to  150°  to  160°  F.  For  three  tunnels  it  will  be  about  10  feet  by 
22  feet.  If  it  is  much  smaller  than  the  dimensions  given  in  the  speci- 
fications it  will  be  unsatisfactory.  The  heating  room  should  be  as 
long  as  the  tunnels  for  the  best  results.  The  furnace  should  be  located 
below  the  lower  ends  of  the  tunnels  in  order  that  the  heat  radiated 
from  the  furnace  will  be  fully  utilized. 

The  furnace  room  should  be  high;  at  least  nine  feet  at  the 
lower  end  and  twelve  feet  at  the  upper  end  of  the  tunnels.  This 
permits  the  use  of  sufficient  heating  pipe  and  causes  the  air  to  be 
thoroughly  mixed  and  of  uniform  temperature  before  it  enters  the 
tunnels. 

Fire-proof  construction  for  the  fire  pit,  as  given  in  the  specifica- 
tions, is  essential.  Cement  is  preferable  to  corrugated  sheet  metal 
because  the  radiation  losses  are  less  and  cement  or  brick  walls  act 
similarly  to  the  walls  of  a  bake-oven  in  stabilizing  the  temperature 
and  in  causing  the  air  to  be  more  uniformly  mixed.  Cement  walls 
in  other  words  act  as  a  reservoir  of  heat. 

The  air-heating  pipes  must  give  enough  radiating  surface  to  impart 
most  of  the  heat  from  the  burning  fuel  to  the  air.  About  150  feet  of 
13-inch  pipe,  or  200  feet  of  9-inch  pipe,  is  desirable  for  three  tunnels. 
It  is  led  back  and  forth  across  the  chamber  and  across  ends  of  the 
chamber  by  T  unions.  The  pipe  must  have  a  gentle  upward  slope 
from  the  furnace  to  the  smokestack.    All  joints  must  be  smoke-tight; 


13 

otherwise  smoke  and  bad  odors  will  enter  the  tunnel  and  spoil  the 
fruit.  The  topmost  pipe  should  be  three  feet  or  more  below  the  tunnel 
floor  to  permit  proper  mixing  of  the  heated  air.  Figure  5  illustrates 
a  heating  chamber  for  four  tunnels. 

The  fresh-air  inlets  must  be  placed  near  the  floor  of  the  heating 
chamber  in  order  that  the  incoming  air  will  flow  over  the  heating 
pipes.  Hollow  tiles  have  been  used  with  good  results.  The  inlets 
must  be  fitted  with  sliding  doors  to  permit  regulation  of  the  air  flow. 

The  furnace  should  be  large  enough  to  burn  completely  the  crude 
oil  or  other  fuel  used.  The  furnace  given  in  the  specifications  will 
take  care  of  a  large  rotary  or  forced  draft  oil-burner  of  such  types  as 
the  Ray  or  Johnson  burners.  A  forced  draft  oil-burner  is  very  satis- 
factory, although  wood  or  coal  may  also  be  used.  If  desired,  the  top 
of  furnace  may  be  covered  with  brick.  The  walls  and  floor  of  the 
furnace  are  best  made  of  fire  brick,  although  heavy  iron  has  been 
used  successfully  by  E.  B.  Stone  of  San  Jose.  Hop  kiln  stoves  are 
often  used  in  Oregon,  but  are  said  to  be  short-lived.  Commercially 
built,  ready-to-install  furnaces  may  be  bought  from  certain  California 
firms.    Most  of  these  are  very  satisfactory. 

The  trays  are  of  wire  screen  of  y3  or  %-inch  mesh ;  %-inch  mesh 
may  allow  small  prunes  to  drop  through.  Wooden  trays  can  not  be 
used  advantageously.  Wooden  trays  impede  air  circulation,  result  in 
a  large  loss  of  heat  used  in  heating  the  wood,  and  permit  drying  on 
one  side  only  of  the  fruit.  The  objections  are  made  to  wire  screen 
trays  that  they  are  expensive  and  that  the  screen  marks  the  fruit. 
Most  of  the  marks,  however,  disappear  during  processing  and  packing. 
The  increased  capacity  and  saving  in  fuel  offset  any  extra  cost  of 
screen  trays. 

A  tray  3'  X  4'  is  as  large  as  one  man  can  handle ;  a  3'  X  3'  tray 
is  more  convenient,  but  to  facilitate  the  transfer  of  fruit  from  8'  X  3' 
dry-yard  trays,  the  3'  X  4'  evaporator  tray  will  be  found  useful. 
Two  of  these  equal  one  wooden  tray. 

Construction  of  a  tray :  Make  a  frame  of  %"  X  l1/^"  spruce,  4' 
long  and  3'  wide.  Cut  a  piece  of  galvanized  wire  screen  (*4"  or  y3" 
mesh)  one  inch  larger  each  way  than  the  frame.  Turn  back  the  edges 
of  the  screen  one  inch  to  give  a  firmer  hold  for  nailing.  Tack  it  to 
the  frame  stretching  it  tightly.  Then  over  the  screen  and  first  frame 
nail  a  second  frame  of  the  same  size  and  construction  as  the  first. 
Nail  two  %"  X  %"  strips  across  the  tray  on  one  side  only  and  one 
foot  from  each  end  to  give  rigidity.  The  tray  is  reversible  and  should 
be  reversed  occasionally  to  prevent  sagging  of  the  screen.  The  frame 
will  be  about  2  inches  thick  and  iy2  inches  wide. 


14 

(d)  Operation  of  the  Evaporator :  To  operate  the  tunnel  evapora- 
tor, proceed  as  follows :  Start  the  furnace ;  open  the  air  intakes  and 
ventilator  dampers.  Heat  the  tunnels  to  150°  F.  at  lower  end.  Place 
fresh  or  wet  fruit  on  the  trays,  about  25  pounds  to  a  3'  X  4'  tray, 
and  fill  the  tunnel  by  sliding  trays  in  from  upper  end.  Regulate  heat 
and  draft  so  that  a  temperature  of  150°  F.  is  maintained  at  the  lower 
end  of  the  tunnels  until  the  prunes  at  this  end  are  dry  enough.  Do 
not  dry  the  prunes  too  much.  Prunes  in  the  evaporator  are  much 
softer  than  prunes  of  the  same  moisture  content  on  trays  in  the 
sun  because  of  the  higher  temperature  in  the  evaporator.  To  make  a 
fair  comparison,  remove  a  few  prunes  and  chill  them  in  the  air  for 
ten  or  fifteen  minutes  and  note  texture  of  the  flesh. 

When  the  trays  of  prunes  at  lower  end  of  tunnel  are  sufficiently 
dry,  remove  them  and  enter  the  same  number  of  trays  at  the  upper 
end  of  the  evaporator. 

The  trays  must  be  so  arranged  that  they  are  offset  over  the  throat 
of  the  tunnel,  as  shown  in  figure  3.  This  permits  even  circulation  of 
the  air. 

2.  Air-Blast  Evaporator. — The  evaporator  to  be  described  consists 
essentially  of  two  horizontal  tunnels  placed  side  by  side  and  resting 
on  the  ground;  a  fan  and  air  heating  system  at  one  end  of  tunnels 
and  ventilator  at  other  end ;  and  of  two  sets  of  car  tracks  upon  which 
loaded  cars  of  trays  are  placed.    The  specifications  are  as  follows : 

(a)   General  Specifications  of  Air-Blast  Evaporator: 

(1)  Two  tunnels,  36'  X  7'  X  7',  resting  on  ground  and  side  by  side.  (See 
fig.  6.)  A  slope  of  %"  or  %"  per  foot  from  entrance  to  fan  end  of  drier  will 
facilitate  handling  of  loaded  cars. 

(2)  Two  sets  of  ordinary  dry-yard  tracks  in  each  tunnel,  36'  long  inside 
tunnel  and  connecting  to  transfer  tracks,  as  shown  in  figure  6. 

(3)  Transfer  track,  15'  beyond  end  of  tunnels  and  at  right  angle  to  tunnels. 
(See  fig.  10.) 

(4)  Transfer  track  connecting  tracks  inside  tunnels  to  yard  tracks  parallel  to 
tunnel.  This  transfer  track  located  36'  from  entrance  end  of  tunnel  and  11'  in 
front  of  fan.      (See  fig.  6.) 

(5)  Folding  doors  closing  ends  of  tunnels  at  entrance  end  of  tunnels;  two 
doors  to  each  tunnel;  each  door  3%'  wide  and  each  pair  of  doors  meeting  at 
center  of  each  tunnel.     (See  fig.  6.) 

(6)  Sliding  door  10'  long,  7'  high  at  side  of  each  tunnel  at  fan  end  to  permit 
removal  of  carload  of  8'  X  3'  trays  crosswise.     (See  fig.  6.) 

(7)  Fan,  6'  disc  fan  to  deliver  35,000  cubic  feet  of  air  per  minute.     275  r.p.m. 

(8)  Motor  for  fan,  8  h.p. 

(9)  Air  heater:  3000  feet  or  more  of  1"  black  pipe  built  into  vertical  coils  and 
connected  to  40  h.p.  boiler  and  return  steam  traps.  These  coils  can  be  bought 
ready  made  or  built  as  shown  in  figure  9. 


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17 

(10)  Alternative  form  of  air  heater:  Furnace  room,  15'  wide,  20'  long,  10' 
high,  of  sheet  metal  or  concrete,  tile  or  brick;  an  8'  X  2%'  X  2'  fire-brick  furnace 
connected  to  a  system  of  300  or  more  feet  of  12"  to  16"  radiating  pipe  and  smoke- 
stack, as  illustrated  in  figure  5,  and  equipped  with  large  rotary  oil-burner.  Fur- 
nace room  to  be  suitably  connected  by  sheet  metal  to  fan  room.  Fan  room  to  be 
of  sheet  metal  or  other  fire-proof  material  to  reduce  fire  risk. 

(11)  Eoof  of  evaporator:  Of  rubberoid  roofing,  corrugated  sheet  iron,  or 
shingles. 

(12)  Trays:  Ordinary  8'  X  3'  dry-yard  trays  may  be  used,  but  better  results 
will  be  obtained  with  *4"  mesh  screen  trays,  3'  X  4'  in  size  with  open  ends  for 
passage  of  air.     About  1000  such  trays  would  be  needed. 


Fig.  8. — Disc  fan.      (Courtesy  of  B.  F.   Sturtevant  Company.) 


(&)  Cost  of  Air-Blast  Evaporator:  A  California  firm  gave  an 
estimate  of  $1550  for  cost  of  fan,  steam  coils,  trap,  and  connections  to 
boiler.  The  alternative  form  of  air  heater  described  in  specification 
10  would  be  cheaper.  A  40  h.p.  boiler  will  cost,  installed  complete 
and  ready  to  operate,  not  less  than  $1000.  The  tunnels,  tracks,  and 
building  should  be  built  for  $1000.  The  motor  will  cost  about  $200 
and  incidental  equipment  should  not  exceed  $250.  On  the  basis  of 
these  rough  cost  estimates,  the  evaporator  would  cost  about  $4000. 

The  West  Side  Association  evaporator  was  built  several  years  ago 
for  about  $1500  without  the  boiler.  At  that  time  the  fan  and  coils 
cost  only  about  $850.  The  association  had  installed  a  boiler  several 
years  before  the  evaporator  was  built  and  has  used  it  for  heating  water 
for  dipping  prunes.  No  record  of  cost  of  the  boiler  could  be  given. 
Boiler,  fans,  pipe,  and  lumber  are  at  present  abnormally  high  in  price. 
In  normal  times,  the  evaporator  should  easily  be  built  and  equipped 
for  $3000,  especially  if  the  alternative  form  of  air  heater  is  used. 


18 

(c)  Discussion  of  Air-Blast  Evaporator:  The  drying  chambers  are 
specified  horizontal.  If  they  are  built  with  a  gentle  slope  from  the 
ventilator  end  toward  the  fan  end,  the  cars  of  loaded  trays  may  be 
moved  much  more  easily  and  it  is  probable  that  the  air  circulation 
will  be  improved.  The  American  Products  Company  of  San  Fran- 
cisco has  found  that  sloping  tunnels  are  much  to  be  preferred. 

The  chambers  are  wide  enough  to  accommodate  two  lines  each  of 
loaded  cars  with  one  foot  of  free  space  to  allow  for  irregularities  in 
stacking  the  trays,  but  are  not  so  wide  as  to  leave  large  channels 
between  the  trucks  or  between  the  trucks  and  walls.  If  the  air  tends 
to  follow  these  channels,  baffles  may  be  placed  to  deflect  air  over  trays. 


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Fig.  9. — Air  heating  steam  coils  of  Yolo  Orchard  Company's  evaporator,  Wood- 
land.    Note  downward  slope  of  pipes  to  facilitate  drainage. 

Each  tunnel  has  two  lines  of  track.  These  tracks  are  connected  to 
yard  tracks  outside  the  dryer  and  to  a  transfer  track  at  dryer  exit, 
as  shown  in  figure  6. 

The  walls  and  ceiling  of  the  tunnels  should  be  as  nearly  air-tight 
as  possible,  although  the  partition  between  the  two  tunnels  need  not 
be  so. 

The  ventilator  should  equal  or  exceed  in  area  of  cross-section  the 
area  of  cross-section  of  the  fan  in  order  that  the  air  flow  shall  not 
be  hindered.  The  current  of  air  from  the  fan  should  flow  in  the 
same  direction  as  the  prevailing  winds.  In  some  evaporators  the  ends 
of  the  tunnels  opposite  the  fan  are  left  open  and  the  spent  air  is 
allowed  to  escape  without  the  use  of  a  ventilator.  Such  a  plan  is 
said  by  some  engineers  to  give  good  results  and  is  much  better  than 
the  use  of  too  small  a  ventilator.  A  good  ventilator  assists  the  fan 
by  increasing  the  air  flow. 


19 

Air  circulation  in  the  evaporator  is  obtained  by  use  of  a  fan. 
A  disc  fan  is  recommended  in  preference  to  a  pressure  blower  because 
the  disc  fan  requires  very  much  less  power  for  operation  and  is  much 
cheaper  than  the  pressure  blower  type.  The  disc  fan  delivers  a  large 
volume  of  air  at  low  .pressure  and  is  not  satisfactory  where  very  heavy 
resistance  opposes  the  air  flow,  but  gives  very  good  results  with  the 


Fig-.  10. — Transfer  car  and  track  used  by  West  Side  evaporator. 


Fig.  11. — Illustrating  method  of  stacking  wooden  trays  on  evaporator  truck. 
Note  that  the  trays  are  offset  and  have  %"  pieces  between  them  to  permit  passage 
of  air. 


style  of  dryer  under  discussion.  The  fan  may  be  bought  ready  to 
connect  or  may  be  made  to  order  by  a  sheet-metal  shop.  A  six-foot 
disc  fan  will  require  an  8  h.p.  motor  or  engine  of  the  same  power. 
A  steam  engine  may  be  used  and  the  exhaust  steam  may  be  passed 
through  the  coils  to  heat  the  air.  This  system  is  used  in  the  West  Side 
evaporator  at  Cupertino.  If  an  electric  motor  is  used  it  is  best  to 
protect  it  against  the  high  temperature  of  the  air  passing  through 
the  fan.     Fig.  6  illustrates  a  small  closed  room  for  the  motor. 


20 

The  fan  draws  the  outside  air  through  a  heating  system.  In  the 
specifications  a  heater  made  of  at  least  3000  feet  of  black  iron  pipe 
connected  to  a  40  h.p.  boiler  is  recommended.  This  heater  may  be 
obtained  from  various  manufacturers  or  may  be  built  by  a  steam 
fitter  and  assembled  at  the  plant ;  the  manufactured  heater  will  usually 
be  the  more  satisfactory.  Figure  9  illustrates  a  home-made  air  heat- 
ing coil. 

Steam  boilers  and  steam  coils  are  very  expensive  and  their  use 
requires  the  employment  of  a  licensed  steam  engineer.     For  these 


Fig.  12. — Wire  screen  trays  used  in  Heilmann  and  O'Brien  Evaporator,  Marys- 
ville,  California.  Note  construction  of  trays  to  permit  passage  of  air.  Also  note 
arrangement  of  overhead  conveyor.  There  is  too  much  space  between  trays  and 
walls. 


reasons,  many  builders  of  evaporators  install  a  furnace  and  heating 
pipes  similar  to  the  heating  system  previously  described  for  the 
Oregon  tunnel  evaporator.  The  furnace  must  be  large  and  at  least 
300  feet  of  12-inch  or  larger  pipe  must  be  installed.  Specification  10, 
page  17,  fully  describes  such  an  air  heater.  Large  drums  have  been 
used  instead  of  pipe  but  do  not  give  sufficient  radiating  surface  and 
result  in  a  very  large  loss  of  heat  through  the  smoke  stack. 

The  total  area  of  the  air  intakes  into  the  heating  chamber  should 
be  as  large  as  a  cross-section  of  the  fan.  They  must  be  fitted  with 
sliding  doors  by  which  the  air  flow  may  be  regulated. 


21 


A  self-registering  thermometer  is  very  useful.  Other  good  ther- 
mometers should  be  installed  at  entrance  and  exit  of  the  tunnels. 

Wooden  trays  may  be  used  if  stacked  on  the  trucks  so  that  air 
may  pass  between  them  freely.  The  evaporator  will  hold  about  320 
such  trays.  (See  fig.  11.)  Wire  screen  trays  3'  X  4'  in  size  may 
be  made  as  follows :  Make  sides  of  2"  X  1"  material,  making  height 
of  sides  2".  Make  ends  of  1"  X  2",  but  place  the  pieces  flat.  This 
makes  ends  1"  lower  than  sides.  Nail  screen  to  bottom  of  the  frame 
and  cover  with  14"  strips.  At  18  inches  from  each  end  of  the  tray 
nail  a  %"  X  %"  strip  above  the  screen  to  prevent  sagging  and  to 


/=?EC£ll//NG  S/iED 


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r/LOOR    PL/IN  (y»o**r*c« 

Fig.  13. — Floor  plan  of  Young  evaporator.     Beceiving  shed  is  not  drawn  to 


scale. 


support  the  frame.  These  trays  are  to  be  stacked  lengthwise  on  the 
tunnel  trucks  to  permit  free  passage  of  air  from  the  fan.  The  evap- 
orator will  hold  about  800  such  trays.  Figure  12  illustrates  another 
style  of  tray  that  may  be  used. 

(d)  Operation  of  Air-Blast  Evaporator:  The  path  travelled  by  the 
trays  from  the  dry-yard  through  the  evaporator  can  be  seen  best  by 
referring  to  figure  6.  A  yard  truck  is  loaded  with  trays,  as  shown 
in  figure  11.  The  truck  is  then  brought  on  yard  track  A  to  transfer 
track  B.  The  car  is  transferred  to  track  B  and  by  means  of  B  to 
one  of  the  tunnel  tracks  C.  The  car  is  then  pushed  into  the  drying 
chamber  on  track  C,  the  long  diameter  of  the  trays  being  parallel  to 
the  tracks  C.  When  the  tunnels  are  filled  with  cars  the  fan  and  steam 
coils  are  started.  The  temperature  of  the  air  delivered  to  the  evapora- 
tor is  regulated  to  150°  F. 


22 

When  the  fruit  on  the  trays  at  the  fan  end  of  the  evaporator  is 
dry  the  car  of  trays  containing  this  fruit  is  moved  to  the  transfer 
track  D.  The  carload  of  dry  trays  is  then  removed  from  the  drying 
tunnel  to  one  of  the  yard  tracks  A.  The  trays  are  taken  crosswise 
from  the  evaporator;  i.e.,  with  their  greater  diameter  still  parallel 
to  the  tunnels.  As  one  car  is  removed  at  D  another  carload  of  fresh 
fruit  is  entered  at  C  and  the  line  of  cars  in  the  tunnel  is  moved  ahead 
to  make  room  for  the  fresh  fruit. 

3.  The  Young  Evaporator. — This  is  a  small,  inexpensive  and 
satisfactory  evaporator  used  in  Napa  County.  The  specifications 
below  are  those  of  the  evaporator  in  the  dry  yard  of  Fred  Haas, 
Yountville.    It  is  a  simple  form  of  stack  evaporator. 

(a)   General  Specifications  of  Young  Evaporator: 

(1)  Four  drying  compartments,  9'  long  X  7'  high  X  37"  wide  above  a  fire 
pit.     Compartments  open  above  pit  and  to  ventilator. 

(2)  Each  compartment  equipped  with  15  horizontal  tray  runways  of  2"  X  2" 
material;  runways  9'  long  and  nailed  to  2"  X  4"  studding. 

(3)  Partitions  between  compartments  open  except  for  tray  runways. 

(4)  One  folding  door,  7'  X  38"  for  each  compartment. 

(5)  Outer  walls  of  dryer  of  1"  X  12"  material  surfaced  one  side,  double,  with 
3"    sawdust-filled    space    between. 

(6)  Roof  of  lean-to  construction,  lower  side  at  height  of  top  of  drying 
compartments,  higher  side  about  4  feet  above  top  of  compartments. 

(7)  Ventilator,  2'  X  3'  and  3'  high  in  center  of  roof.  (Should  be  about 
10'  high.) 

(8)  Fire  pit  14'  long  X  9'  wide,  2'  below  ground  line  and  4'  above.  Walls  of 
6"  concrete.  Several  1'  X  1'  openings  at  ground  line  for  admission  of  air.  Would 
be  better  if  air  were  admitted  at  bottom  of  pit  instead  of  at  ground  line. 

(9)  Furnace,  4'  long  X  3'  wide  X  2'  high  inside.  Lined  with  fire  brick. 
Furnace  door  about  8"  X  12".  Excavation  and  steps  leading  from  ground  level 
to  furnace  floor  level. 

(10)  Burner,  larger  stove  distillate  type.     Fed  from  tank  above  by  gravity. 

(11)  Air  heating  pipes:  One  16"  pipe  connecting  to  furnace  at  one  end  of 
chamber  and  to  20"  drum  at  other  end.  This  20"  drum  in  turn  connects  to  two 
9"  pipes  which  return  to  the  furnace  end  of  room.  At  the  furnace  end  of  room 
these  two  pipes  connect  by  vertical  elbows  to  two  9"  pipes  which  return  to  opposite 
end  of  pit.  They  are  here  joined  by  a  T  joint  to  a  9"  pipe  returning  above  center 
of  pit  to  the  smokestack.  There  are  in  all  one  16"  pipe  about  8'  long;  one  20" 
drum,  about  &/%  long;  and  five  lengths  of  9"  pipe,  each  about  13'  long.  One  end 
of  each  9"  pipe  projects  through  the  cement  wall  and  is  covered  with  a  soot 
cap  to  facilitate  cleaning. 

(12)  Trays:  Ordinary  8'  X  3'  dry-yard  trays  are  used  but  4'  X  3'  screen  trays 
of  the  form  described  for  the  Oregon  tunnel  evaporator  would  be  much  better. 


23 


24 

(&)  Cost  of  Young  Evaporator:  The  above  evaporator  without 
trays  can  probably  be  built  for  $500  or  less.  The  receiving  shed  is 
not  included.  Mr.  Fred  Haas  of  Yountville  can  give  information 
relative  to  costs. 

(c)  Discussion  of  Young  Evaporator:  This  evaporator  would  be 
improved  if  the  capacity  of  the  furnace  were  increased  and  if  wire 
screen  trays  were  used  instead  of  the  ordinary  wooden  dry-yard  tray. 


1 

" 

Fig.  15. — View  of  Young  evaporator.     The  small  box-like  structure  resting  on  the 
cement  walls  is  the  evaporator.     The  large  building  is  the  receiving  shed. 


The  capacity  of  the  evaporator  is  sixty  8'  X  3'  trays  of  fruit  per 
charge.  If  two-thirds  dry  when  it  enters  the  evaporator,  the  fruit  will 
dry  in  about  twelve  hours.  Mr.  Haas  was  able  to  dry  two  charges  of 
rain-damaged  fruit  per  twenty-four  hours  and  during  the  season 
handled  about  100  tons  of  prunes  on  dry  basis.  The  prunes  were  in 
some  cases  sulfured  after  the  rains  to  prevent  moulding  and  fermen- 
tation until  they  could  be  artificially  evaporated. 

An  evaporator  similar  to  the  above  is  located  in  the  Fisher  dry- 
yard  at  Union  Station,  Napa  County.     This  dryer  is  equipped  with 


25 

two  car  tracks  and  holds  four  loaded  cars  of  8'  X  3'  trays  at  each 
charge.  It  is  said  to  have  given  good  results.  Both  evaporators  were 
installed  by  the  Young  Hardware  Company  of  Napa. 

(d)  Operation  of  Young  Evaporator:  The  furnace  is  first  started 
and  the  dryer  heated  to  140°  to  150°  F.  The  trays  of  fruit  are  placed 
in  the  compartments  by  sliding  them  in  on  the  tray  runways.  If 
wooden  trays  are  used,  drying  is  hastened  and  made  more  uniform 
by  stirring  the  fruit  occasionally.  As  the  fruit  becomes  dry  the  trays 
are  removed  and  are  replaced  by  fresh  trays.  Two  men  are  needed 
to  handle  8'  X  3'  trays. 

A  piece  of  sheet  metal  about  4'  wide  and  14'  long  is  hung  above 
the  radiating  pipes  to  distribute  the  heat  evenly. 

About  thirty  gallons  of  stove  distillate  is  used  per  twenty-four 
hours. 

4.  Other  Evaporators. — 

(a)  Kiln  Evaporator:  This  evaporator  is  used  in  California 
chiefly  for  drying  hops,  but  is  used  occasionally  for  apple  drying. 
It  is  the  most  common  form  of  apple  evaporator  in  New  York.  It 
may  be  built  as  a  single  evaporator  or  as  a  battery  of  several 
evaporators. 

It  is  built  in  two  stories.  The  upper  story  houses  the  drying  floor. 
This  floor  is  usually  20'  X  20'  and  is  made  of  narrow  wooden  strips 
with  i/4"  or  %"  space  between  them.  The  spaces  permit  the  passage 
of  air.  Over  the  drying  floor  is  a  steep  four-sided  roof  which  has  at 
its  apex  a  large  ventilator  for  the  escape  of  the  hot  air.  The  pre- 
pared fruit  or  vegetables  are  spread  on  the  drying  floor  and  turned 
by  a  fork  or  shovel  during  drying.  The  lower  floor  contains  the 
heating  system.  This  consists  of  a  wood-  or  oil-burning  furnace  from 
which  is  led  a  series  of  several  turns  of  heating  flues  or  pipes.  These 
flues  are  usually  joined  at  the  furnace  by  a  T  and  may  be  led  around 
beneath  the  drying  floor  several  times. 

Drying  is  accomplished  by  the  hot  air  rising  from  the  hot  flues 
through  the  spaces  in  the  drying  floor.  The  air  is  carried  off  by  the 
ventilator  in  the  roof. 

Caldwell*  places  the  cost  of  a  four-kiln  evaporator  with  wooden 
building  at  about  $2500.  For  a  complete  description  of  this  type  of 
evaporator  see  Farmers'  Bulletin  903,  United  States  Department  of 
Agriculture.  This  style  of  evaporator  is  not  well  adapted  to  prune 
drying,  as  there  is  a  tendency  for  the  product  to  mold  because  of  the 
slow  drying;  the  control  of  temperature  during  drying  is  not  satis- 
factory, and  the  fruit  is  mashed  and  broken  during  turning.     It  is 

*  Bulletin  148,  University  of  Washington  Agricultural  Experiment  Station. 


26 


.  27 

not  recommended  for  prune  drying,  but  where  already  installed  for 
other  purposes  could  be  used  in  an  emergency. 

(6)  Watsonville  Stack  Evaporator:  This  type  of  evaporator  is 
used  commonly  in  California  for  apple  drying.  The  heating  system 
is  the  same  as  for  the  kiln  and  tunnel  evaporators.  The  drying  cab- 
inets or  "stacks"  are  on  the  second  floor.  The  drying  takes  place  on 
trays  which  slide  into  a  drying  chamber  situated  directly  above  the 
heating  flues.  The  bottom  of  the  stack  is  open ;  the  top  consists  of  an 
inverted  hopper  which  ends  in  a  tall  ventilator.  According  to  infor- 
mation furnished  by  Mr.  Henry  Washburn,  Farm  Advisor,  this  style 


Fig.  17. — Anderson  Barngrover  evaporator  on  Mr.  Husted's  place  near  Saratoga. 
Note  that  evaporator  is  placed  on  side  hill. 


of  evaporator  can  be  built  for  $2500  to  $3500  per  unit.  Each  unit 
will  produce  from  y2  to  %  ton  of  dried  apples  per  day  or  will  take 
about  seven  tons  of  fresh  apples  per  day.  Mr.  Washburn  can  furnish 
specifications  for  this  evaporator  and  the  names  of  builders.  The 
evaporator  is  inefficient  in  use  of  heat.  It  has  no  satisfactory  tem- 
perature control  and  often  rises  to  too  high  a  temperature. 

(c)  The  Anderson  Evaporator:  This  is  a  form  of  tunnel  evapora- 
tor found  in  a  number  of  dry-yards  in  the  Santa  Clara  Valley.  It  is 
used  with  the  ordinary  8'  X  3'  wooden  tray. 

Figures  16  and  17  will  give  an  idea  of  its  construction.  As 
originally  installed,  it  had  very  inadequate  heating  and  ventilating 
capacity.  Several  were  used  during  the  past  season,  but  none  with 
entire  satisfaction.  Mr.  C.  B.  Husted,  near  Saratoga,  installed  stove- 
distillate  burners  in  the  furnaces  and  then  obtained  fair  results,  in 
finishing  prunes  that  were  almost  dry  when  the  rain  came.  His 
evaporator  is  shown  in  figure  17. 


28 

E.  B.  Stone  has  installed  the  most  satisfactory  improvement  seen 
for  this  evaporator.  He  has  built  a  furnace  about  8  feet  from  the 
evaporator.  It  is  in  a  pit  about  5'  deep  X  4'  wide.  The  furnace 
is  of  iron,  is  8'  X  12"  and  has  2"  ribs.  It  connects  to  several  long 
turns  of  12"  pipe  which  pass  beneath  the  throats  of  the  tunnels.  The 
furnace  is  enclosed  in  an  arched  fire-proof  chamber.  The  radiating 
pipes,  12'  long  X  12"  in  diameter,  are  beneath  the  lower  ends  of 
tunnels  in  a  cement  pit  about  5'  deep  and  4'  wide.  Air  is  blown  over 
the  furnace  and  pipes  by  a  No.  2  Sturtevant  fan,  is  heated  to  200°  F., 
enters  the  lower  ends  of  the  tunnels  and  is  forced  over  the  trays  by 
the  fan.  Stove  distillate  is  used  as  a  fuel  and  is  burned  by  a  Johnson 
Whirlwind  burner.    The  cost  of  fuel  is  30c  per  hour. 

In  the  writer's  opinion,  the  fan  used  should  be  larger  and  the  heat 
introduced  at  about  160°  F.  rather  than  200°  F.  Wire  screen  trays 
should  be  used  instead  of  wooden  trays.  For  four  Anderson  tunnels 
a  5'  disc  fan,  5  h.p.  motor,  150'  of  12"  pipe  and  a  large  burner  could 
be  used. 

If  wire  screen  trays  3'  X  4'  were  used  in  all  Anderson  evaporators 
now  installed  and  if  the  heating  capacity  were  increased,  as  Mr.  Stone 
has  done  or  by  building  furnace  pits  about  8'  deep  X  15'  long  beneath 
the  tunnels  and  installing  the  heating  system  described  for  the  Oregon 
tunnel  evaporator,  good  results  could  be  obtained.  So  long  as  wooden 
trays  are  used,  dissatisfaction  is  almost  certain  to  result. 

(d)  Commercially  Built  Evaporators  :  Many  very  satisfactory  and 
reliable  evaporators  are  built  and  sold  by  California  firms  and  by 
others.  Some  of  these  are  of  standard  size  and  construction  while 
others  are  built  to  order.  Evaporators  may  be  had  for  from  about 
$400  upward.  Names  of  manufacturers  can  be  furnished  by  the 
University  upon  request. 

SUMMARY  AND  CONCLUSIONS 

1.  An  evaporator  is  a  good  investment  for  the  average  California 
prune  dry-yard.    The  past  season  has  proved  its  utility. 

2.  In  designing  an  evaporator,  provision  must  be  made  for  (a) 
sufficient  heat  production,  (b)  sufficient  radiating  surface,  (c)  good 
air  circulation,  and  (d)  control  of  temperature.  Most  evaporators 
now  in  use  are  deficient  in  one  or  more  of  these  factors.  Efficiency 
of  heat  utilization  and  convenience  in  operation  are  also  desirable. 

3.  The  Oregon  tunnel  evaporator,  an  air-blast  evaporator  for 
wooden  trays,  and  the  Young  evaporator  have  been  described  in  detail. 
General  descriptions  have  been  given  of  the  kiln  evaporator,  Watson- 


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ville  stack  evaporator,  and  the  Anderson  evaporator.  An  installation 
by  which  the  capacity  of  the  Anderson  evaporator  may  be  increased 
has  been  described. 

RECOMMENDATION 

For  the  small  dry-yard  the  Young  type  of  evaporator  or  an 
evaporator  of  similar  size  is  recommended.  The  Oregon  tunnel 
evaporator  would  answer  the  needs  of  the  average  size  yard.  Where 
wooden  trays  are  to  be  used,  the  air-blast  evaporator  will  be  best. 


